This invention relates generally to plastic bag fabricating machines and more particularly to systems for controlling operation of such machines.
Various machines exist for automatically fabricating plastic bags with economy and speed. Typically, these machines operate by drawing a predetermined length of plastic web from a supply roll and thereafter transversely cutting and thermally sealing the web to form a bag.
Frequently, it is desired to have labels, instructions or similar graphic material appear on the finished bags. To this end, the graphic material is printed onto the plastic web in regularly spaced locations or fields, and it is necessary to ensure that the web is cut and sealed only at precise locations between the fields in order to insure proper placement of the printed matter on the finished bag.
One technique for ensuring that the web is cut and sealed at the proper locations is to advance the web by a predetermined distance, or draw length, equal to the spacing of the printed fields. However, minor errors, resulting from stretching or shrinkage of the web as it is printed and wound onto and off of the supply roll, accumulate and gross misalignment can occur over time as the bag fabricating machine operates. Another technique for ensuring that the web is cut and sealed at the desired locations between adjacent printed fields is to print regularly spaced indexing marks or "eyemarks" on the web before the web is fed into the bag fabricating machine. An optical sensor detects the passage of each eyemark by a predetermined location and signals the machine when to stop, cut and seal the web. However, it is often desired to include printed matter between. successive eyemarks. In order to avoid sensing other marks which appear to be eyemarks, it is preferred to enable the optical sensor only over a small interval or "window" in which the eyemark is expected to be seen.
This technique is effective in correcting minor systematic errors in alignment (such as, an intermittent lengthening or shortening of the actual distance between eyemarks). Progressive errors can arise as a result of varying tensions as the web is wound and unwound from the supply roll, and this can result in the eyemarks falling outside of the sensing window. These types of errors can cause improper alignment of the printed matter on the finished bags.
In view of the foregoing, it is a general object of the present invention to provide a new and improved plastic bag fabricating machine.
It is a more specific object of the present invention to provide a new and improved system for controlling the operation of a plastic bag fabricating machine so as to ensure proper alignment of printed matter on the finished bags.
It is a still more specific object of the present invention to provide a system for controlling the operation of a plastic bag fabricating machine so as to compensate for progressive variations between the nominal spacing of printed matter fields on the web and the actual spacings of the fields on the webs.